In the field of radiology, breast imaging is considered to be one of the most demanding in terms of resolution and contrast. Specialists in the field are interested in imaging lesions or masses that may require an imaging aperture that is about 50 microns in size or less. At the same time, contrast requirements are also demanding since the lesions or masses to be imaged sometimes have x-ray absorption characteristics similar to that of the surrounding tissue. Moreover with heightened interest in early detection, image resolution and contrast demands are continuing to increase.
X-ray mammography has been performed using both film-based and digital systems. In film-based systems, x-rays are transmitted through the patient's breast and impinge upon a phosphor screen. Light emitted from the phosphor screen, due to excitation by the impinging x-rays, is used to expose a light sensitive film. The film is then developed to yield an image of the patient's breast which can be viewed on a light box. In digital systems, a light sensitive receiver is used in place of the film. The receiver yields an electronic signal which can be processed for real-time viewing on a high-resolution monitor.
Improvements in film-based, x-ray imaging systems have resulted in improved image resolution and exposure of the patient to lower radiation dosages. Film-based systems are subject, however, to certain limitations. For example, film granularity and film screen noise limits the spatial resolution of the resulting image. Moreover, films which produce higher resolution images generally require greater radiation doses. Additionally, the resulting image contrast can be significantly affected by scattered radiation. Although the effects of scattered radiation may be reduced by using an anti-scatter grid, the latter necessitates a greater radiation dosage.
Digital systems are advantageous in that the above described problems involving film granularity and film screen noise are avoided while theoretically being capable of providing outstanding image resolution. In addition to high resolution, digital imaging systems provide other advantages including the ability to manipulate various processing and display parameters for an image, once the image has been stored, to optimize its display. Provisions for real-time imaging capability are also advantageous in that procedures such as, for example, biopsies may be performed while viewing a real-time image of the tissue site with biopsy instruments applied thereto.
The ability of a digital system to image, or replicate, a given area may be represented as a modulation transfer function (MTF). The MTF ranges in value from 0 to 1 wherein a value of 1 represents a theoretically perfect image duplication of the area while a value of 0 represents no duplication of the area of interest. Many factors may contribute to deterioration of the MTF in a digital system including, but not limited to, mechanical tolerances, frictional implications of interfacing mechanical components and design/production variabilities in the digital camera being used. Of course, the objective in producing imaging systems is to provide the highest possible MTF in a cost-effective manner.
One recent advance in digital imaging systems utilizes a scanned receiver in a time delay integration (TDI) mode. This technique is described in detail in U.S. Pat. No. 5,526,394, which is assigned to the assignee of the present invention and is incorporated herein in its entirety by reference. In brief, the TDI receiver is scanned across a region of interest during its exposure to a radiation signal. At the same time, drive signals are provided to the receiver such that electrical charge is incrementally accumulated, or integrated, by the receiver and output from the receiver as imaging data. The scanned TDI approach provides significant advantages over prior art systems including, but not limited to, reducing the effects of scattered radiation, providing for full field breast imaging and reducing the required radiation dosage.